Bal­lots and on­line vot­ing forms were due to be sent to party mem­bers, who will have un­til Septem­ber 21 to decide whether to re­place Cor­byn with MP Owen Smith, 46, who was pre­vi­ously lit­tle known out­side West­min­ster.

Smith pre­cip­i­tated the elec­tion by declar­ing his can­di­dacy last month, warn­ing that Cor­byn’s lead­er­ship was mak­ing the pos­si­bil­ity of a split in the party “danger­ously real”.

Smith, a for­mer mem­ber of Cor­byn’s top team, is also tar­get­ing vot­ers to the left of the party, promis­ing a “so­cial­ist revo­lu­tion”.

“Not some misty-eyed, ro­man­tic no­tion of a revo­lu­tion where we are go­ing to over­throw cap­i­tal­ism and return to a so­cial­ist nir­vana ... but a cold-eyed, prac­ti­cal so­cial­ist revo­lu­tion where we build a bet­ter Bri­tain,” he said last month.

Cor­byn is favourite to stay as leader, re­tain­ing the sup­port of most trade unions and many who signed up last year to pro­pel him to a shock win in the lead­er­ship elec­tion called after the party’s thrash­ing in the gen­eral elec­tion.

But he has failed to win over many of the party’s MPs, 80 per­cent of whom backed a re­cent vote of no con­fi­dence in their leader. The re­sult of the lead­er­ship con­test is to be an­nounced on Septem­ber 24 dur­ing a spe­cial congress in Liver­pool, north­west Eng­land.

Bri­tain’s vote on June 23 to leave the Euro­pean Union pro­vided the cat­a­lyst for the lead­er­ship chal­lenge, with many in the party crit­i­cis­ing Cor­byn’s per­for­mance dur­ing the cam­paign.

Last month, Labour MP An­gela Ea­gle said she was with­draw­ing from the race to top­ple Cor­byn, say­ing she would give her sup­port to Smith to boost the chances of a lead­er­ship change. — AFP